MTBR is normally quoted for components
which are economically viable to repair should they fail. Whereas MTBF
is normally quoted for components for which it is not economically
viable to repair and which would therefore normally be replaced once
they have failed.

For example a hard disk would not
normally be
considered repairable should it fail, and thus manufacturers may quote
a MTBF figure. W. There will a level of repair where the
replaceable entity is not repairable, hence the "R" could be
extrapolated to not only mean "repair"
but also "replacement".
Also
starting with MTBF (Mean Time Between Failure, or the time between
faults occurring), if the system under consideration has
built-in
redundancy then it will not necessarily mean that the system has to be
repaired immediately; indeed it may not be possible in say an aircraft.
It depends on whether one wishes to use the calculations for the
purposes of spares provisioning or resourcing a repair facility; hence
BER (Beyond
Economic Repair) comes into play.